<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .one {
      /* font-size: 20px; */
      width: 2em;
      background-color: red;
    }
    .two {
      font-size: 30px;
    }
  </style>
</head>
<body>
  <div class="father">
    <div class="one">555</div>
    <div class="two"></div>
  </div>
</body>
</html>